The average IQ score, by definition, is 100. People who take IQ tests are compared to each other, and the average score is placed at 100.
However, IQ scores change over time. While some make the argument that IQ scores are immutable and can't be changed by external factors, Jim Flynn has shown that IQ scores have risen in much of the world since approximately 1930.
There is no single IQ test, and individuals might have significantly different scores on different tests. Although scores between different tests tend to correlate fairly well, it's possible for someone who scores below average on one type of test to score above average on another.